1. Field of the Invention
The present invention is in the field of tanks, principally for underground fluid storage such as fuel storage, and is particularly directed to secondary fluid containment to protect against leakage in the region of tank pipe fittings and connections.
2. Description of the Prior Art
Regulations in many states and of the U.S. federal government currently require double wall construction for underground fluid storage tanks such as fuel storage tanks, in order to provide secondary fluid containment because of environmental considerations. Such double wall tank construction constitutes, in effect, an inner tank supported within an outer tank. The inner tank defines the primary, inner chamber which provides primary containment for fluid being stored, while the space defined between the inner and outer tanks defines a secondary chamber which provides for secondary containment of the fluid in the event a leak should develop through the wall of the inner tank, as for example from corrosion, a faulty weld, or seismic or other mechanical stressing. One or more fluid-sensing monitors are conventionally located in communication with one or more low zones in the secondary chamber between the two tanks, and any leakage from the inner, primary tank chamber into the secondary chamber is directed toward one or more of such monitors which then provide an alarm signal indicating the leakage.
There are several different grades and types of underground storage tanks generally considered in the art to be of double wall construction which are currently in use in the United States, and these are almost all of cylindrical construction and adapted to be layed on their sides underground, i.e., have their cylindrical axes disposed generally horizontally. A full double wall tank consisting of two complete cylindrical tanks, one inside the other, is designated a "double wall" tank. This type of tank has double end walls and 360.degree. double cylindrical wall protection. Another type of tank which is not completely of double wall construction but is nevertheless commonly referred to as a double wall tank is known as a "wrap tank." In the wrap tank, the primary fluid holding tank is cylindrical, with an outer sheet provided which gives double wall protection for approximately 330.degree. around the lower part of the tank, leaving the top part of the tank with only the single wall protection of the primary tank. The wrap tank is utilized with the consideration that the greatest potential for failure of the primary tank is in its lower part, with a relatively small potential for failure at its top. While regulations of some states still allow use of the wrap tank as a double wall tank, other, more progressive states such as California require the full double wall tank.
Underground storage tanks for hazardous and flammable materials such as fuels require access pipe fittings which extend through the top of the tank from the outside into the primary containment chamber within the tank, and most of such fittings have function pipes connected thereto which extend upwardly to surface equipment. Typically for the storage of fuels such as gasoline and diesel fuel, there are at least five such function fittings required, including a fill fitting, a turbine fitting for fluid extraction, a fitting for gauging, a vent fitting, and a vapor recovery fitting.
In a full double wall metal tank, these pipe fittings conventionally extend through both walls at the top of the tank, being welded or otherwise bonded to both the outer and inner tanks. While this arrangement technically has full double wall protection at the fittings, in practice it has the serious disadvantage that the connections of function pipes to the fittings, as well as the lengths of such pipes and pipe joints therein, conventionally have only single wall protection, and therefore do not fulfill the intent and spirit of current governmental regulations requiring double wall containment. Such conventional pipe fittings in full double wall metal tanks have the further disadvantage that the welds joining the fittings to the inner tank must either be made from the inside of the inner tank or through an oversize access hole in the outer tank which then must be filled.
Wrap tanks are metal tanks, and for such tanks the pipe fittings are conveniently welded into the single wall at the top of the tank. This, of course, provides only single wall containment at the fittings, and again, there is only single wall containment where function pipes are connected to the fittings and along the lengths of such pipes. Accordingly, such wrap tanks are even further away than the full double wall tanks from complying with the intent and spirit of current governmental regulations.
Many tanks currently being produced and sold as full double wall tanks are nonmetallic, as for example having a wound filament fiberglass/resin construction. The filament winding fabrication process does not permit fittings to be integrally incorporated in such a double wall nonmetallic tank when the two walls are being fabricated, and if the fittings were to be disposed directly through the two walls of the nonmetallic double wall tank, then it would be necessary to drill holes through the two walls and bond the fittings to the walls from both the inside and outside, which would be costly, difficult and unreliable. Therefore, for nonmetallic double wall tanks the almost universal current procedure is to provide manway openings, and to provide metal covers for such openings through which the pipe fittings are welded. Normally only three fittings can be accommodated in a single manway cover, so that where more than three fittings are required, which is usually the case, two manways and associated metal covers with fittings are conventionally provided with double wall nonmetallic tanks. Such metal manway covers with fittings for nonmetallic tanks are conventionally of single wall construction, lacking any double wall containment at the fittings, and again, lacking any double wall containment where function pipes connect to the fittings or along the lengths of such pipes.
Applicant's U.S. Pat. No. 4,685,585, issued Aug. 11, 1987, teaches the incorporation of pipe fittings with a pair of spaced covers of a double wall manway system. While this affords true double wall containment in the manway and at the fittings, it again does not provide double wall containment where function pipes connect to the fittings, or along the lengths of such pipes.
Thus, despite current governmental regulations requiring double wall containment for underground tanks, many so-called double wall tanks currently actually provide only single wall protection at pipe fittings and along the lengths of pipes connected to the fittings, and applicant is not aware of any prior art system which provides true double wall containment for all regions associated with a double wall tank, including the pipe fittings per se, connections of function pipes to the pipe fittings, and along the lengths of such pipes extending to surface equipment.
There are currently strong indications from the EPA (U.S. Environmental Protection Agency) and other regulatory agencies that statistically, in the field, for underground tanks which are properly installed, whether double wall or single wall, the failures which occur are at the connections to the tanks and not in the tanks themselves.